It is possible to convert wind energy to electrical energy by using a wind turbine to drive the rotor of a generator. A plurality of wind turbines can be connected together in clusters to form a wind farm.
There is a growing tendency for wind farms to be located offshore because of environmental issues and concerns. In this situation, the power generated by the wind turbines must be transmitted to shore and this is typically achieved using subsea cables. Such subsea cables can make use of high-voltage direct current (HVDC) power transmission.
In a conventional point to point power transmission system of the type shown in FIG. 1, wind turbine clusters 102 are connected to offshore collector platforms 104 and 104′. Each collector platform 104, 104′ includes a busbar 106, 106′ that is connected to the busbar 108 of a separate offshore converter platform 110. The connection between the busbars 106, 106′ of the collector platforms and the busbar 108 of the converter platform 110 is by means of medium voltage (MV) or high voltage (HV) ac cabling 112 and step-up transformers 114, 114′ which are provided on the offshore collector platforms.
The offshore converter platform 110 includes an AC/DC converter 116 having ac terminals that are connected to the busbar 108 by means of a converter transformer 118 and dc terminals that are connected to the upstream ends of first and second dc transmission lines 120a, 120b implemented as HV dc cabling. The downstream ends of the first and second dc transmission lines 120a, 120b are connected to the busbar 122 of an onshore converter station 124 by means of a DC/DC converter 126. In particular, the dc terminals of the DC/DC converter 126 are connected to the first and second dc transmission lines 120a, 120b and the ac terminals of the DC/DC converter are connected to the busbar 122 by means of a converter transformer 128. The busbar 122 is then connected to a utility grid.